I have the same problem. I just discovered it this afternoon. After doing some research, I found that it may be what is called a mastoid osteoma.
I Have A Hard Lump On The Tip Of My Mastoid Bone Directly Behind My Right Ear Lobe Its Not Painful But If I Push On It Its Tender. It Seems To Move Very Slightly If I Push On It. Its About Three Quarters Of The Size Of A M&m. It Almost Feels Like A?